APUs are well known and commonly used in aircraft for the purpose of generating power to be used by various elements within the aircraft. For example, APUs can be utilized to provide power to a refrigerant compressor in the aircraft via a generator so that cabin air cooling can be provided when the main engine system of the aircraft is not running. Likewise, APUs can be used to provide power to various electrical systems of the aircraft via a generator in order to ensure that devices such as air-circulating fans and cabins lights can be activated when the main engine system is not operating.
A typical APU contains a combustor which receives compressed air from an air compressor and fuel from a fuel supply source in order to generate a high pressure combustion product (such as a hot gas) for driving a turbine, and a gearbox or transmission for transferring power from the shaft of the turbine to a generator. Associated with the APU are a variety of electronic devices for performing functions such as power control and switching. Fuel is supplied to the combustor for combustion therein, and lubricating oil is supplied to elements disposed within or associated with the APU that have moving parts (such as the gearbox and/or the generator) for lubrication and cooling purposes.
As is the case with many well known electronic devices and components, electronic devices associated with an APU must be cooled to maintain the electronic devices at a temperature within a range in which they can safely operate, i.e., in which they can operate without risk of damage or malfunction. Common methods of cooling electronic devices associated with an APU include oil (conductive and sprayed) cooling and air cooling. Furthermore, as exemplified by U.S. Pat. No. 3,623,564, the use of engine fuel in the cooling of electronic equipment mounted on a gas turbine engine is also known. Additionally, U.S. Pat. No. 3,071,185 discloses that the temperature of a fuel circulating in a temperature control system for aircraft equipment can be lowered by passing the circulating fuel through a fuel input heat exchanger and can be raised by passing the circulating fluid through a hydraulic oil input heat exchanger.
While the above-mentioned methods are useful for controlling the temperature of a variety of different elements, such methods are considered to be less than satisfactory for cooling APU electronic devices which have different temperature ranges within which they can operate. For example, in a fuel-driven oil-lubricated APU system utilizing high power electronic devices which can safely operate at temperatures up to 250.degree. F. and low power electronic devices which can only safely operates at temperatures up to 200.degree. F., neither the fuel-based cooling method nor the cooling method utilizing oil or air can be satisfactorily utilized. If the fuel is used to cool both the high and low power electronic devices, the heat exchange between the fuel and the high power electronic devices, because such devices generally dissipate large amounts of heat, may cause the fuel to be raised to an unsatisfactory temperature level at which vapor lock or other undesirable phenomena can occur. On the other hand, if either oil or air is used to cool both the high and low power electronic devices, the size, weight, and cost of the APU system is increased, along with the amount of energy required by the APU system, due to the nature of the equipment required to maintain such coolants at an acceptable temperature. For example, if the oil which lubricates the gearbox and/or the generator is used for cooling both the high and low power electronic devices, an oil cooler which can cool the oil to a low temperature suitable for cooling the high power electronic devices must be utilized. As a result, due to the requirement that the oil cooler be able to cool the oil to temperatures lower than those suitable for cooling the low power electronic devices, a larger, heavier and more expensive oil cooler which utilizes a greater amount of energy is required, thereby increasing the amount of energy utilized by the APU system and the size, weight and cost thereof.